<!DOCTYPE html>
<html>
	<head>
		<meta charset="utf-8">
		<title></title>
	</head>
	<body>
		<button onclick="fun()" style="color: red;">123</button>
		
		<script type="text/javascript">
			var fun = function(){
				var button = document.querySelector("button");
				 var str = button.innerHTML;
				 str = "<b>"+str+"</b>";
				 button.innerHTML=str;
			}
		</script>
	</body>
</html>
